Showing papers on "Slip ratio published in 1998"

Traction control of electric vehicle: basic experimental results using the test EV "UOT electric march"

Abstract: The most distinct advantage of the electric vehicle is its quick and precise torque generation. However, most electric vehicles developed to date have not yet utilized this feature. In this paper, two novel traction control techniques of an electric vehicle using this advantage are proposed. One is the model-following control and the other is the optimal slip ratio control. The basic effectiveness of the proposed methods is demonstrated by real experiments using the DC-motor-driven test vehicle "UOT (University of Tokyo) Electric March".

Slip velocity and slip layer thickness in flow of concentrated suspensions

Abstract: The rheological characterization of highly filled suspensions consisting of a Newtonian matrix (hydroxyl-terminated polybutadiene), mixed with two different sizes of aluminum powder (30% and above by volume) and two different sizes of glass beads (50% and above by volume), was performed using a parallel disk rheometer with emphasis on the wall slip phenomenon. The effects of the solid content, particle size, type of solid particle material, and temperature on slip velocity and slip layer thickness were investigated. Suspensions of small particles of aluminum (mean diameter of 5.03 μm) did not show slip at any concentration up to the maximum packing fraction. However, suspensions of the other particles exhibited slip at the wall, at concentrations close to their maximum packing fraction. In these suspensions, the slip velocity increased linearly with the shear stress, and at constant shear stress, the slip velocity increased with increasing temperature. The slip layer thickness increased proportionally with increasing size of the particles for the glass beads. Up to a certain value of (filler content/maximum packing fraction), ϕ/ϕm, the slip layer thickness divided by the particle diameter, δ/DP, was 0, but it suddenly increased and reached a value that was independent of ϕ/ϕm and the temperature. On average, the ratio of δ/DP was 0.071 for aluminum and 0.037 for glass beads. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 515–522, 1998

Slip in Entangled Polymer Melts. 1. General Features

Abstract: Apparent violations of the no-slip boundary condition are studied using a series of narrow molecular-weight distribution polybutadiene melts (67300 ≤ Mn ≤ 650000), subjected to plane-Couette shearing over clean silica glass surfaces Simultaneous measurements of slip velocity and shear stress reveal several new molecular characteristics of slip in entangled polymers log−log plots of slip velocity versus shear stress display three distinct power-law regimes: (i) A weak slip regime at low shear stresses that is characterized by extrapolation/slip lengths b of the order of a few micrometers; (ii) A stick-slip regime at intermediate shear stresses marked by periodic oscillations in slip velocity and shear stress; (iii) A strong slip regime beyond a defined critical shear stress σ* Slip violations in this last regime are characterized by large slip velocities and massive extrapolation lengths (b∞ ∼ 100−1500 μm) For all polymers studied the critical stress σ* for the weak-to-strong slip transition is found

Effects of slip, slip rate, and shear heating on the friction of granite

Abstract: The stability of fault slip is sensitive to the way in which frictional strength responds to changes in slip rate and in particular to the effective velocity dependence of steady state friction Δμss/Δ ln V. This quantity can vary substantially with displacement, temperature and slip rate. To investigate the physical basis for this behavior and the possible influence of shear heating, we slid initially bare granite surfaces in unconfined rotary shear to displacements of hundreds of millimeters at normal stresses, σn of 10 and 25 MPa and at room temperature. We imposed step changes in slip rate within the range 10−2 to 103.5 μm/s and also monitored frictional heating with thermistors embedded in the granite. The transient response of μ to slip rate steps was fit to a rate- and state-dependent friction law using two state variables to estimate the values of several parameters in the constitutive law. The first 20 mm of slip shows rising friction and falling Δμss/Δ ln V; further slip shows roughly constant friction, Δμss/Δ ln V and parameter values, suggesting that a steady state condition is reached on the fault surface. At V ≤ 10 μm/s, Δμss/Δ ln V = −0.004 ± 0.001. At higher rates the response is sensitive to normal stress: At σn = 25 MPa granite shows a transition to effective velocity strengthening (Δμss/Δ ln V = 0.008 ± 0.004) at the highest slip rates tested. At 10 MPa granite shows a less dramatic change to Δμss/Δ ln V ≈ 0 at the highest rates. The maximum temperature measured in the granite is ∼60°C at 25 MPa and 103.5 μm/s. Temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogeneous frictional heating over the sliding surface. The simplest interpretation of our measurements of Δμss/Δ ln V is that the granite is inherently velocity weakening (∂μss/∂ ln V 0 mimics velocity strengthening. These results have implications for the frictional behavior of faults during earthquakes. High slip rates may cause a switch to effective velocity strengthening which could limit peak coseismic slip rate and stress drop. For fluid-saturated faults, strengthening by this mechanism may be partly or fully offset by weakening due to thermal pressurization of a poorly drained pore fluid.

Slip of Newtonian fluids at slid boundary

Abstract: A real fluid does not slip at the surface of a solid boundary. Most experimental results of a Newtonian fluid satisfy with this condition. If a real fluid can slip freely over the surface of a solid boundary, how can we deduce the slip velocity. The purpose of this study is to experimentally clarify fluid slip velocity of Newtonian fluids at the duct wall. Velocity profiles of tap water and 20 wt% glycerin solution flowing in a square duct with a highly water repellent wall were measured by means of a conical hot film anemometer. Consequently, the velocity profile with the slip is measured, and the laminar drag reduction phenomena and the friction factor formula for a square duct with fluid slip at the wall have been obtained

Slip flow in rectangular microtubes

Abstract: Microscale fluid dynamics has received intensive interest due to the extraordinary advances in electronic device miniaturization, where the peaks of temperature from hotspots must be reduced by a coolant flowing in a microchannel. One of the most meaningful microscale effects is the emergence of slip flow. The present analysis is concerned with the 2-D velocity distribution of steady-state, hydrodynamically developed, laminar slip flow, for Newtonian fluids in rectangular ducts. The solutions describing velocity profiles, friction factors, shear stresses, momentum flux, and kinetic energy correction factors are derived resorting to the integral transform technique. The results are shown as functions of the aspect ratio and the Knudsen number, in the limit of Kn 0 they reproduce perfectly the well-known results of macroscale fluid dynamics.

A model for slip at polymer/solid interfaces

Abstract: A steady-state dependence of slip velocity on shear stress at polymer/solid surfaces is derived from a molecular model that includes effects of drag on polymer chains, disentanglement, and detachment and reattachment of chains from the solid surface. The dependence includes two turning points and thus a region where slip velocity is a multivalued function of shear stress. This behavior is consistent with experimental observations of abrupt and hysteretic transitions between regimes of small and large slip. Parameters involved in the model are estimated and a reasonable comparison with experiment is obtained.

Wall slip in polymer melts: A pseudo-chemical model

Abstract: A chemical-type theory for wall slip in polymer melts is developed by modeling the exchange of bridging sites between two opposing polymeric and solid surfaces. Kinetic equations, describing surface coverage by bridging monomers, are formulated and analyzed to evaluate the stability of adhesive contact and slip characteristics of the viscoelastic melt. Order of magnitude estimates of the kinetic coefficients suggest that the polymer–solid interface is always at equilibrium, even under slip. The model displays the following features. The polymer slips at all stresses; the slip velocity, vs, obeys time-free volume superposition and depends on both shear and normal stresses. At small stresses, vs is linear in shear stress and proportional to a function of the work of adhesion; the slip parameter b (the slip extrapolation length scale) takes on the same form as that proposed by de Gennes, but displays an additional dependence on adhesive energy. At constant vs the shear stress is proportional to the adhesive ...

The effect of molecular mass and temperature on the slip of polystyrene melts at low stress levels

Abstract: The slip of monodisperse polystyrene melts next to a solid, inhomogeneous, metal substrate (stainless steel) is measured at small stress levels for a variety of temperatures. A critical stress, below which no slip occurs, is not seen and the polystyrene melts used here slip at all stress levels. The slip velocity is quantified by the slip length (b, equal to the slip velocity divided by the shear rate) and friction coefficient (k, equal to the slip velocity divided by the shear stress). The slip length shows complicated dependence with both the molecular mass and temperature, however, when converted to the friction coefficient a master curve with molecular mass results for temperatures above 170 °C. The data are compared to contemporary theories for slip. It is concluded that none of the present theories accurately represent the data and that the number of adsorbed molecules are in a dynamic equilibrium which affects the slip behavior. Analysis of the force on the adsorbed molecules during shear demonstrates this may cause adhesive failure which contributes to the slip.

Traction control system of vehicles combining feedback control with feedforward control

Abstract: A traction control system of a vehicle having a device for calculating a target traction torque of each of a pair of driving wheels based upon operating conditions of the vehicle, a device for calculating a target slip ratio of each of the pair of driving wheels based upon the target traction torque calculated therefor, and a device for controlling the engine and the brake system such that actual slip ratio of each of the pair of driving wheels coincides with the target slip ratio calculated therefor according to a feedback control, with a partial feedforward control of the engine and the brake system based upon the target traction torque

Developments of fretting sliding criteria to quantify the local friction coefficient evolution under partial slip condition

Abstract: The determination of the sliding conditions is of primary importance in describing the loading conditions in a fretting contact. Cracking is mainly observed under partial slip condition [1] whereas gross slip situation with high dissipated energy promotes wear and debris formation. To quantify such sliding conditions, various adimensional criteria have been recently introduced. They allow the quantification of the transition between partial slip to gross slip for sphere/flat contacts through various constants [2]. This study focuses on recent developments, where the sliding criteria are studied, taking into account the time evolution. This permits the formalisation of the concept of fretting regime [3] and more particularly the mixed regime which corresponds to sliding transitions during the test. Moreover the local friction coefficient in the partial slip external annulus can be estimated by combining the sliding ratio with the tangential force evolution. This fundamental study of friction is compared to experiments on a sphere/flat situation with steel contacts. Theoretical local friction coefficient analysis is then compared to local observations in the wear scar obtained under partial slip. The local friction value is related to various distributions of strong metal/metal shear stress interactions and more accommodative debris layers. It is shown that the mixed slip regime is associated with a decrease of the contact dissipated energy which indirectly interacts on the extension of the stick domain and then an increase of the local friction coefficient in the annular sliding zone.

Wall slip of a polydimethylsiloxane extruded through a slit die with rough steel surfaces: Micrometric measurement at the wall with fluorescent-labeled chains

Abstract: Upstream instability and wall slip of a high molecular weight polydimethylsiloxane (PDMS) have been studied in a rough stainless-steel slit die. The velocity field at a micrometric scale close to a wall is determined with a fluorescence technique. A film of a mixture of PDMS and small fluorescent-labeled chains (PDMS–NBD) is deposited on the steel surface prior to the high-pressure flow. During the flow, the fluorescence of a small area in the middle of the surface is excited with an argon laser line and measured with a phototube. The signal decays towards an asymptotic low value, due to stray light and thermal current. The slip velocity is inferred from a comparison between the experimental decrease and a theoretical model, which takes diffusion effects into account. It is shown that the upstream instability induces a transverse oscillating velocity in the channel. A precise determination of the residual fluorescence after the wall slip, with a known diffusion coefficient and surface roughness, shows tha...

Boundary layer over a slotted plate

Abstract: In order to investigate the boundary layer over a slotted plate, we first set up a hydrodynamic boundary condition for a parallel flow from a numerical study, performed on a periodic perforated medium at the local scale of the perforations. This boundary condition links the shear stress at the wall and the slip velocity along the plate by a partial slip coefficient. Then, a parametric study is conducted to analyse this partial slip coefficient versus the surface porosity of the plate and the Reynolds number of the flow. Afterwards, we introduce this partial slip boundary condition in the Blasius formalism and express the dynamic boundary layer thickness, the reduced slip velocity and the shear stress at the wall as functions of the slip coefficient, the Reynolds number and the reduced abscissa on the plate. Finally, we compare these latter laws with direct simulation results of the boundary layer flow over a slotted plate, considered as a very thin homogeneous permeable flat wall.

Vehicle behavior control device

Abstract: An upper limit of a target braking force for performing behavior control, and an upper limit of its variation rate, are limited so as not to produce hunting due to interference with an anti-skid control A microcomputer comprising a braking force controller, in a step 81, calculates a target slip ratio (S 1* ) before limiting from a target braking force (F 0* ) before limiting and a wheel braking rigidity coefficient (ks), and when it is determined in a step 82 that (S 1* ) is larger than a maximum controllable slip ratio (Smax), in a step 84, limits a target slip rate (S 2* ) after limiting to (Smax) and limits the upper limit of target braking force after limiting, calculated by multiplying (S 2* ) by (ks), to a maximum value at which slip control is possible In a step 85, a differential (ΔS*) between (S 2* ) and a target slip ratio (S 0* ) on an immediately preceding occasion is taken as a target slip variation rate, and when it is determined in a step 86 that (ΔS*) is faster than a predetermined value (ΔS 0 ), the slip ratio (S*) is determined, in steps 89, 90, according to the target slip variation direction determined in a step 87, so that the variation rate of the target slip ratio (S*) is equal to or less than (ΔS 0 ) In this way, the upper limit of the variation rate of the target braking force calculated by multiplying (S*) and (ks), can be limited

Slip ratio antiskid controller using mu/slip ratio generated velocity reference

Abstract: An antiskid brake controller utilizes measured wheel speed in order to provide brake control for a vehicle such as an aircraft. The controller estimates the speed of the vehicle via approximation based on the measured wheel speed and a model of the mu-slip ratio curve representing the wheel to running surface friction characteristics. The controller then predicts the slip ratio based on the measured wheel speed and estimated vehicle speed. The difference between the predicted slip ratio and a predefined desired slip ratio is used to drive a modified integral controller segment to achieve maximum obtainable friction. In another embodiment, the controller measures and integrates the applied braking torque in order to estimate the vehicle speed. The estimated vehicle speed is again combined with the measured wheel speed to determine an estimated slip ratio. The controller compares the estimated slip ratio with a predefined desired slip ratio which again drives a modified integral controller.

Effect of pressure-dependent slip on flow curve multiplicity

Abstract: Various microstructural pictures for slip at polymer/solid interfaces lead to relations which have a region where multiple values of slip velocity are predicted for the same shear stress. This leads to the expectation of multivalued flow curves, which has been verified in specific cases by numerous researchers. We study the effect of pressure dependence on flow curve multiplicity using a simple multivalued slip relation to model the phenomena of hysteresis and spurt flow in polymer extrusion. A continuation technique is used to trace out the boundaries of the region of flow curve multiplicity as pressure drop and die length to diameter (L/D) ratio are changed. Results for Newtonian, shear thinning and viscoelastic constitutive equations show that, despite the multivalued nature of the slip model, multiplicity (and thus hysteresis) is absent at high L/D. For the sake of completeness, we also carry out time-dependent simulations at constant piston speed taking fluid compressibility into account. These simulations show that oscillations in the pressure drop and exit volumetric flow rate result only if the system is operated in the multiplicity region of the steady state flow curve, in agreement with the results of similar simulations by researchers using various multivalued slip models without pressure dependence. The results demonstrate that a multivalued slip model does not guarantee multiplicity in the flow curve for the constant pressure drop operation, nor oscillations for constant piston speed operation.

Motion control device for vehicle

Abstract: PROBLEM TO BE SOLVED: To achieve more favorable motion control of a vehicle by computing a target value for the lateral force of a front wheel and a target value for the lateral force of a rear wheel instead of a target value for the lateral force of the vehicle. SOLUTION: Target longitudinal force Fxa, front wheel lateral force Fyfa, rear wheel lateral force Fyra and moment Ma as the sums of the longitudinal force, front wheel lateral force, rear wheel lateral force and moment of a vehicle and target longitudinal force Fxt and target moment Mt with the slip ratio Si of each wheel being zero are computed (S250), micro-coefficients showing changes dFx, dFyf, dFyr, dM in the longitudinal force, front wheel lateral force, rear wheel lateral force and moment with a micro-change dSi in the slip ratio are computed, correction amounts δFx, δFyf, δFyr, δM of the longitudinal force, front wheel lateral force, rear wheel lateral force and moment are computed in accordance with differences between each of target values and each of actual values and the micro-coefficients by convergence computation where the weighting of the rear wheel lateral force is greater than that of the front wheel lateral force (S200-350), and correction amount δSi of the slip ratio is computed to achieve the correction amount. The braking force of each wheel is controlled by using a target slip ratio obtained by correcting the former target slip ratio with δSi (S400-550). COPYRIGHT: (C)2004,JPO

Linear stability diagrams for the shear flow of an Oldroyd-B fluid with slip along the fixed wall

Abstract: We consider the time-de- pendent shear flow of an Oldroyd-B fluid with slip along the fixed wall. Slip is allowed by means of a gen- eric slip equation predicting that the shear stress is a non-monotonic function of the velocity at the wall. The complete one-dimensional sta- bility analysis to one-dimensional disturbances is carried out and the corresponding neutral stability dia- grams are constructed. Asymptotic results for large values of the elasticity number and finite element calculations are also presented. The instability regimes are within or coincide with the negative-slope regime of the slip equation. The numerical calculations agree with the linear stability results when the size of the initial perturbation is small. Large perturbations may de- stabilize a linearly stable steady state, leading to a periodic solution. The period and the amplitude of the periodic solutions increase with elasticity.

Basic Study on Traction Control of Electric Vehicle

Abstract: Recent researches of electric vehicle are normally focused on batteries and motors themselves. However electric vehicle can achieve high performance motion control by taking advantage of its excellent controllability. In this paper, we propose “traction control”. First, we clarify the static characteristic of vehicle slip phenomena by introducing “constant slip ratio curve”. Next, we propose “model following control” and “slip ratio control” and show their effectiveness by experiments using a real test vehicle.

The reduction of viscous extrusion stresses and extrudate swell computation using slippery exit surfaces

Abstract: Wall slip has been pointed out as a way to reduce cracks and swelling on polymer extrudates. To investigate this phenomenon, the capillary extrusion of a purely viscous generalized Newtonian fluid is computed with the finite element code POLYFLOW with a realistic slip law. This fits with the friction curve data for a polydimethylsiloxane (PDMS) in a steel die. It is based on molecular dynamics theory and contains a critical stress below which there is qualitative adhesion at the wall. The existence of a slip boundary condition is shown to modify the morphology of the velocity field which tends toward a plug flow. It thus largely reduces the fully developed stress level and the exit stress concentration. Localized slip at the die exit appears even for fully developed flow under shear rate values which are lower than the critical shear rate corresponding to the occurrence of slip at the wall in a Poiseuille flow. This is explained by exit stress concentration. The effect of slip length is then examined: it is found that exit stress and extrudate swell can be largely reduced by using a very short slip length only. This result shows the interest of optimising slippery surfaces in the design of extrusion processes.

Steady-state flow controlled by the velocity of grain-boundary migration

Abstract: A model is developed for steady-state plastic flow in which the strain rate is controlled by the rate of grain boundary migration and the strain is due to dislocation slip within the grains.

Numerical simulation of the transition from adhesion to slip with friction in generalized Newtonian Poiseuille flow

Abstract: The axisymmetric Poiseuille flow of a purely viscous generalized Newtonian fluid under rate of flow controlled conditions is studied with a change in the boundary conditions at a transition point from an adhesive to a slip condition with friction at the wall. The friction law used originates from an experimental study by (J.M. Piau and N. El Kissi, J. Non-Newtonian Fluid Mech. 54 (1994) 121–142) using a capillary made of steel and a silicone fluid, and is based also on a molecular dynamics theory by (Yu. B. Chernyak, A.I. Leonov, Wear, 108 (1986) 105–138). It gives a non-linear multivalued dependance of the wall shear stress to the velocity at the wall. Moreover, wall shear stress values may become smaller than values obtained when adhesion prevails in the capillary. The shear stress must over-step some limiting stress level to trigger the wall slip. After checking slip boundary condition implementation for the case of Poiseuille flow with slip along the entire wall, the convergence and the validity of the computation was studied. Important morphologic changes of the flow field and the stress field appear around the transition point from adhesion to slip boundary condition. Slip at the wall allows the principal stress difference to be drastically reduced, except in the vicinity of the transition point where this difference is maximum. A peak in shear stress located upstream of the transition, and a peak in elongational stress located downstream of the transition, are observed at the wall. Fully developed near plug-like flows are obtained within about 1D only downstream of the transition point. It is concluded that the effect of slip on extrudates distorsion should appear clearly even when the exit slippery zone is reduced to 1D.

Attitude control device of vehicle

Abstract: PROBLEM TO BE SOLVED: To improve the accuracy in attitude control by estimating the slip ratio between a wheel and a road surface in a traveling direction of a vehicle by an arithmetic means to obtain the force in a longitudinal direction and that in a lateral direction to be applied to each wheel SOLUTION: An attitude control device 1 takes the outputs of a yaw rate sensor 6 and a steering angle sensor 14 when the breaking operation is detected by a brake pressure sensor 12, and determines the wheel speed V' on the basis of the wheel rotating speed sensor 10 of the front and rear wheels 8, 9 Then it takes the detecting output Gx from a longitudinal acceleration sensor 18 to calculate the slip ratio sf in a tire force arithmetic part 2 The actual vehicle speed V is calculated by using the obtained value Simultaneously, the slip ratio of each wheel is calculated on the basis of the actual speed V and the rotating speed of each wheel to be used as the longitudinal force Fx Further each angle of side slip βf, βr of the front and rear wheels are determined on the basis of the actual speed V, the acceleration Gy in a lateral direction, the yaw rate ω, the steering angle δ or the like, and the force Fy in the lateral direction is determined on the basis of the braking force or the driving force acting on each wheel

A correlation for drop size and slip velocity in reciprocating plate column

Abstract: Drop size distributions, mean drop diameter and slip velocity are experimentally studied in a reciprocating plate column The correlations for the above are presented in terms of plate geometry, flow rates of phases and physical properties of the system covering a wide range in experimental conditions